Hello guys I'm Guido from Italy .
I'm happy to discover this forum.
I have a question about an upgrade of my headphone amp Pro-Ject Head Box II to improve frequency response, sound, ecc...
The biggest trouble is substitution of op-amp that is an JRC2068 : what is the best choice op-amp for substitution ?
NE5532 or NE5534 , OPA1641 , OPA 2134 , LME 49723 , or other ?
For the Manufacter ? Texas Instr, National , Burr Brown , Analog Devices or
others ? Is it advisable cooling these op-amp ?
Do you think the new op-amp upgrade increase the temperature of other components , example in alim section in voltage regulator ?
For other components like capacitors I see Elna or Panasonic FM Caps and maybe I make an upgrade also of ext alim 16 V with other with more power ?
Can you give me some suggests ?
Thanks
Guido

1. Forget about simple parts swapping for now. That rarely results in any tangible improvements when you've got a relatively modern concept with decent parts quality and construction (which applies to this amp).
2. Trace out the schematic, or at least determine the values of several components. Expect something similar to an Eaton amp, modified for single supply operation. If it's got an external 16 V supply, the regulator in there probably is an 7812?

Parts of interest would include:
* Output series resistors. The Head-Box II is reputed to be on the higher side in terms of output impedance, so that's definitely something that could be improved depending on what the resistor values turn out to be (I'd guess something in the 22..100 ohm range). The effect would vary depending on headphones. Even picky fullsize cans tend to be happy once they see less than 10 ohms, so I'd install 4.7 or 5.6 ohm jobs. But let's see what's in there now.
* The buffer stage's emitter resistors, and number of diodes. That would allow determining approximate idle current. (Of course you can always measure the voltage drop across the emitter resistors once their value has been determined.)
* Gain-setting resistors, more out of curiosity.

The concept unfortunately requires output coupling capacitors, and if I recall the inside shots correctly, the unit comes with 220 µF ones stock. That's a perfectly decent value, though if you can make some 470µ bipolar(!) ones fit "somehow", those should be an improvement in measured perforrmance (low-frequency distortion) at least.

Thanks guys for your precious anwser and suggests.
To agdr for the link: many interesting article in that site,long to read but I shall read it all.
To sgrossklass for his suggests for modding.
About this I tell you some things :
- regulator is a 7815 !!then if I have read well datasheett of JRC2068 and NE5532 is enough to supply both op-amp
- Below You can read all modify that I read here :Random Acts of Upgrades: The Upgraded Pro-Ject Head Box Mk II
-1000uF caps were upgraded from 16V to 25V. This is more of a longevity issue than sound.
-Input caps C13, 14, 16 and 17 were upgraded from 4.7uF to 22uF. For two reasons. First, they are almost the smallest FM caps I could find. Second, I wanted to beef up the bass response.
-Output caps C8 and C10 were upgraded from 470uF to 1,000uF. Again, to improve the bass response at the headphones.
-Op-Amp replaced with an NE5532. I know I dissed it as old, but it was a good old op-amp and I have them lying around.

So you know because I have think that these modify op-amp and other are not expensive and possible first the op-amp NE5532 instead of JRC2068
Don't you think this is right ?
BYE
Guido

1) Replaced the output caps with 1000 µF Panasonic FC
2) Replaced the (electrolytic) input caps with film caps
3) Installed a socket for the opamp (now LM4562)
4) Replaced the filter and bypass caps for the opamp

Remarks:

1) This is the only modification that is really needed,
especially if you use low impedance headphones.
2) The input caps after the volume pot (and the output
caps are needed to block DC (because of the single
supply). The input caps before the volume pot could
be ommited (shorted) if you are sure none of your
sources has DC on the output. (Such DC will ruin the
the volume pot.)
3) Use a dual opamp which is unity gain stable (NE5532,
LM4562, LM49720 ...) or leave the JRC2068 in place.
Personally I had never the "night and day difference"
some people report when trying different opamps.
(I have yet to hear an opamp that beats competent made
discrete class a circuity.)
4) Most likely not needed, but I had the parts at hand and
while I´m at...

The volume pot is crap and could be replaced by something
better, but I´m not sure if it is worth it. (Mounting Pirelli
racing tires on a cheap car does not make a Ferrari out of it).

Wow gk7 ,are oyu author of the ariticle in those blog ?
If yes, it's fantastic this is what I'm look for.
The board of my Pro-Ject Head Box MKII has little 2 differences :
1-vol pot no solded on board directly , but on an little adapter
2-I ahve only VD4 diode empy VD3 and VD2 and a VD1 haa his two terminal wired.
Can you have mofify also diode section ? Diodes seems to be 1N40404 , is it right ?
Is necessary also modify diode section ?
Can you tell me also Manufacter reccomended for rach componenent and where I can bur them on web ?
Se my attachment. to understand what I mean.
BYE
Guido

Sorry but I need for explanations about your upgrade
1- what' the reason for substitution of the electrolytic input caps with film caps ? Electrolitic are polarized film no . What type and manufactures for film cpas ?
2- 4) Replaced the filter and bypass caps for the opamp. Sure you are more expert than me e nad maybe you have also electric shema of Head Box II but please can you tell me exactly what it the filter and caps for the opamp ? For by pass you mean unsolder this caps or other ?
Excuse me I am not an expert , be patience with me.
Thanks
Guido

At your board Project has changed this from full
wave rectification to a single diode. I have no
idea why, they are only a few cents.
I would change this to four diodes, but maybe someone
else has an idea why this might have been changed.

Maybe they had issues with heat dissipation in the regulator (e.g. resulting solder fatigue)? Note how inconsistent its mounting seems to be. They originally intended it to be screwed to the board for good heat transfer and mechanical stability.

Do any of you guys have a picture of the bottom side of the PCB? Most of the signal routing seems to be going on there.

So far, R1 and R2 seem to be current limiting resistors between supply/ground and output driver collectors.
R15/17/18/20 (plus another set of same values on the other channel) include two 3R3 (emitter resistors?), one 10R (?) and one 15k (feedback?). 3R3s for emitter resistors would mean about 100 mA of quiescent current. Those power transistors should get nice and toasty @ 750 mW each. With the full-wave rectifier, the regulator would then have to dissipate about 800 mW, which explains the (planned) heatsinking setup.

@ gk7
Thanks for suggests but what mean exactly with " and bypass caps for the opamp " ?
@ sgrossklass
On attachment rear of my board .
Differences between mine and gk7 board can allow me to modify also diode section , then ?
Changing op-amp from JRC2068 to NE5532 or LM4562 or LM49720 ecc. we will have problem of power voltage and cooling of op-amp ?
See attacment.
BYE
Guido